Tungsten disulfide is a solid inorganic lubricate typically applied as a dry film to provide lubrication under conditions that are generally unsuitable for most organic-based lubricants, e.g. high loads, high temperatures (to 500° C. in air) and vacuum environments. Tungsten disulfide powders may be used as an lubrication-enhancing additive in various greases, oils, or self-lubricating polymers. Examples of these applications are described in U.S. Pat. Nos. 4,075,111, 4,715,972, and 5,013,466.
Commercially available tungsten disulfide powders are composed generally of irregularly shaped flat platelets as shown in FIG. 1. It has been postulated that these irregular platelets have chemically reactive edges which cause them to stick to machinery parts and undergo undesirable chemical reactions. Spherical fullerene-like tungsten disulfide nanoparticles have been shown to improve the tribological properties of tungsten disulfide. Such particles are described in International Application No. WO 01/66462 A2. The fullerene-like nanoparticles were made by sulfidizing WO3 in a solid-gas reaction with H2S. The temperature in the reaction path ranged from 750° C. to 850° C. The size and geometry of the WS2 particles was found to be determined by the size and geometry of the WO3 particles being reduced. Larger oxide precursor particles (about 0.5 μm) were slower to convert necessitating the addition of an extra annealing step at 950° C. to complete the conversion. Although larger particles were thought to be a better lubricant in cases where the mating surfaces had higher surface roughness, the process described therein was limited to producing spherical particles up to 0.5 μm.